Add configurable default dispatcher

[twinn]

Summary

Adds a :dispatcher option to child_spec/start_link that sets the default dispatcher module for all broadcast functions. Defaults to Phoenix.PubSub (preserving existing behavior). Can be overridden per-call by passing a dispatcher to broadcast/4 and friends.

{Phoenix.PubSub, name: :my_pubsub, dispatcher: MyApp.Dispatcher}

Motivation

The custom dispatching feature is powerful, but in practice it's difficult to use as a default for an application. The current options are:

1. Pass the dispatcher explicitly on every broadcast call
2. Create a wrapper module that injects the dispatcher

Both add boilerplate for what is conceptually a per-PubSub configuration. The wrapper approach is particularly fragile. Any code that calls Phoenix.PubSub.broadcast/3 directly instead of going through the wrapper bypasses the custom dispatcher entirely. In a growing codebase with multiple contributors, this is easy to miss.

The use case I ran into was filtering broadcasts in test environments, scoping delivery so that concurrent async tests sharing a topic don't leak messages to each other. A configurable default makes this a one-line change at startup rather than a module-level abstraction that every callsite must remember to use.

Implementation

The internal meta :pubsub tuple is extended from {adapter, name} to {adapter, name, dispatcher}. Single Registry.meta lookup per broadcast call, same as before. No additional lookups.

Changes

• Phoenix.PubSub.Supervisor accepts :dispatcher option, stores it in the meta tuple
• All broadcast functions default dispatcher to nil, read the configured default from the meta tuple, and fall back to it when no explicit dispatcher is passed
• 6 new tests covering default dispatcher, per-call override, and all broadcast variants

[josevalim]

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